This invention relates to tools for making microcube retroreflective elements for use in manufacturing retroreflective articles, and in particular, retroreflective sheeting; to articles and sheeting having microcubes; and to methods of making such tools and articles. This invention further relates to tools, articles, and methods wherein said microcubes may have boundary shapes other than triangular.
Microcube retroreflective sheeting is now well-known as a material for making reflective highway signs, safety reflectors, reflective vests and other garments, and other safety-related items. Such retroreflective sheeting typically comprises a layer of a clear resin, such as for example, an acrylic or polycarbonate or vinyl, having a smooth front surface and a plurality of retroreflective microcube elements on the reverse surface. Light incident on the smooth front surface passes through the sheeting, impinges on the retroreflective elements, and is reflected back out through the smooth front surface in a direction nominally 180.degree. to the direction of incidence.
The reverse surface of the resin layer bearing the microcubes may be further provided with additional layers, such as metallization, which enhances the entrance angularity of the sheeting, or hydrophobic silica, adhesives, release liners, or other layers which otherwise contribute to the functionality of the sheeting.
Cube corner retroreflectors have been used on automobiles and for highway markings since the early 1900's. These prior art devices were based on macrocube corner elements made by the pin making art. From the use of macrocubes, a number of optical principles involving cube corner technology have been published, and some have been patented. Generally, these principles have involved changes in the size, shape or tilt of the cube faces, or of the included dihedral angles between faces, to achieve desired retroreflector performance. These known optical principles have included:
increasing the efficiency of the retroreflector at large observation angles by changing one or more of the three dihedral angles of the cube, as taught in Heenan U.S. Pat. No. 3,833,285; PA1 increasing the efficiency of the retroreflector at large incident angles by inclining the cube axis with respect to the normal (often called "angled reflex"), taught, for example, in Leray U.S. Pat. No. 2,055,298 and Br. 423,464, and in Heenan U.S. Pat. No. 3,332,327; PA1 increasing entrance angularity in one or more planes by including in the array cubes with cube axis cant, as taught in Heenan U.S. Pat. No. 3,873,184 and Heenan U.S. Pat. No. 3,923,378, and, in particular, by positioning one face of each of the oppositely oriented cubes more parallel to the front face of the reflector, as taught in Heenan et al U.S. Pat. No. 3,541,606 to increase entrance angularity in two planes at right angles to each other; PA1 increasing uniformity of retroreflectance versus orientation by rotating some cubes by varying degrees about a normal to the front surface of the article, and also by assembling them in arrays of variant dispositions, as in Uding Canadian Pat. No. 785,139; and by angling the cube axis in combination with multiple rotations, as in U.S. Pat. No. 3,923,378.
While these retroreflective optic design principles are well-known in the cube corner art, in more recent years some have attempted to patent them again in microcube sheeting technology, apparently because those persons either did not know what was done in prior macrocube technology, or chose either to ignore or to limit the applicability of the prior art teachings when applied to microcube retroreflective sheeting.
Prior to applicants' present invention, virtually all microcube sheeting has been limited to the use of microcubes made by ruling along parallel planes. This limitation is a result of the microcube dimensions being smaller than the dimensions obtainable by the cutting, polishing and lapping techniques used in the pin making art. The need to use traditional ruling techniques has inhibited the application of known optical principles to microcubes, and has, with one exception, further generally limited percent active aperture to less than 100%.
The present invention is a major advance in microcube sheeting technology. It enhances both the applicability to microcubes of prior known retroreflective optic principles and the manufacturability of microcubes of different base configurations. Before detailing these advances, further background information is provided.
Retroreflective sheeting and methods of forming the microcube retroreflective elements in such sheeting are disclosed, for example in Pricone et al. U.S. Pat. No. 4,486,363, assigned to the common assignee herein, and incorporated herein by reference in its entirety. As disclosed in such patent, the resinous layer of the sheeting may be on the order of 0.01 inch (0.25 mm) thick or less, and the retroreflective elements formed in the reverse face of the resinous layer comprise triangular microcubes such as are known in the manufacture of flexible retroreflective sheeting.
To manufacture such microcube sheeting, generally a master plate of retroreflective triangular microcubes is made by ruling a pattern of retroreflective cube corners into a planar surface of the plate. This is taught generally by Stamm U.S. Pat. No. 3,712,706; is mentioned in U.S. Pat. No. 5,122,902; and is also taught in detail in U.S. Pat. No. 4,478,769, assigned to the applicants' assignee and incorporated herein by reference in its entirety.
As shown in FIGS. 1A, 2 and 3 of the '769 patent, the planar surface of a master plate is ruled with a diamond tool which cuts a series of precise parallel V-shaped grooves. To rule equilateral triangular microcubes, three sets of parallel grooves intersecting one another at angles of 60.degree. are made; each groove also will have an included angle of substantially 70.53.degree., and will be ruled to a groove depth determined by the height of the microcubes desired. This automatically results in an array of oppositely oriented pairs of equilateral triangular microcubes on the face of the master.
The ruled master may then be used to make a series of duplicates, such as by electroforming, and the duplicates are assembled together to form a single "mother" tool. The assembled "mother" tool is used to electroform molds, which are then assembled and ultimately used to form a tool capable of providing the microcube retroreflective elements on the sheeting, such as by embossing, casting, or other means known in the art. A continuous embossing method is disclosed in the aforementioned U.S. Pat. No. 4,478,769; a casting technique for forming microcubes is disclosed, for example, in Rowland U.S. Pat. Nos. 3,684,348 and 3,689,346.
As will be described hereafter, triangular microcubes having bases other than equilateral triangles have been used in an effort to achieve enhanced entrance angularity by use of the well known optical principles taught in macrocube technology. Thus, as taught in applicants' assignee's commonly assigned patent Montalbano U.S. Pat. No. 4,633,567, variations of the triangular microcube may be achieved by changing the tool ruling angles (thus, canting the cube axis), thereby adopting and applying some of the prior optical principles to microcube technology. For example, it is possible to achieve arrays having different entrance angularity or orientation angularity (c.f. Rowland U.S. Pat. No. 3,684,348, col. 10, 11. 1-18 and Montalbano U.S. Pat. No. 4,633,567, col. 6, 11. 4-36)
As previously noted, U.S. Pat. No. 3,833,285, discloses that the observation angularity of cube corner retroreflection can be increased in one plane by increasing (or decreasing) one of the three dihedral angles of the cubes; U.S. Pat. Nos. 3,873,184 and 3,923,378, disclose an array of retroreflective elements wherein the cube axes of neighboring cubes are inclined with respect to each other and oppositely oriented such that the entrance angularity is increased; U.S. Pat. No. 3,541,606 discloses that if one cube face of each of the oppositely oriented cubes is "more parallel" to the front surface, entrance angularity is increased in two planes at right angles to each other. Each of the foregoing patents is incorporated herein by reference.
The identical optical principles used in macrocubes for enhancing retroreflectivity have also been applied to the triangular microsized cubes such as are used in retroreflective sheeting. Thus, U.S. Pat. No. 4,588,258 to Hoopman discloses a retroreflective article with purportedly novel wide angularity wherein an array of triangular microcube elements comprises sets of matched pairs with the cube axes of the cubes in each pair being tilted toward one another; but this simply duplicates the face-more-parallel structure disclosed, for example, in applicants' assignee's prior U.S. Pat. No. 3,541,606, U.S. Pat. No. 3,923,378 or U.S. Pat. No. 3,873,184 patents. Moreover, the Hoopman matched pairs of triangles are inherent when ruling triangles, which at the time of Hoopman's application was the only technique used for manufacturing microcubes.
Similarly, U.S. Pat. No. 4,775,219 to Appeldorn, et al., discloses a retroreflective article of modified observation angularity having an array of microcube retroreflective elements formed by three intersecting sets of parallel V-shaped grooves, wherein at least one of the sets includes, in a repeating pattern, at least two groove side angles that differ from one another. The Appeldorn article merely achieves, in an obvious manner, the identical principle taught years ago in applicants' commonly assigned U.S. Pat. No. 3,833,285.
However, all triangular cubes, while providing adequate retroreflectance, suffer the known disadvantage that inherently by their geometry no more than 66% of their area can be retroreflective for any particular incidence angle. In an attempt to overcome this deficiency of triangular cubes, the Minnesota Mining and Manufacturing Company, in a series of published PCT applications (WO 95/11463; WO 95/11465; WO 95/11467; WO 95/11470), has disclosed arrays of microcubes including some non-triangular cubes, and techniques for ruling such arrays. However, the disclosed arrays have cubes of greatly different heights (which may pose manufacturing problems) and greatly varying aperture size (affecting diffraction and impacting on retroreflectivity). At best, the disclosed arrays provide calculated percent effective aperture (at 0.degree. incidence) of 91%, which appears to fall to about 87% when manufacturing draft is considered (see, e.g., WO 95/11470, FIG. 12). If the cubes are canted by the disclosed ruling technique, the efficiency drops even further. The very nature of forming these cubes by intersecting ruled grooves parallel to a single plane inherently limits the results which can be obtained.
The advantages of the techniques and articles of the present invention, as compared to those obtained by the earlier, triangular microcubes or even by the more recent ruled mixtures of triangular and non-triangular cubes, are shown in the drawings of this application and are more specifically described hereinafter.
Unlike triangular cube corners, hexagonal and rectangular cube corners have the advantage that 100% of their area can be retroreflective even at large incidence angles. Also unlike triangular microcubes, however, hexagonal and rectangular microcubes are not defined by continuous straight lines that extend along a planar surface, and therefore cannot be ruled with intersecting sets of parallel lines all parallel to a common plane. Thus, with the sole exception of the rectangular cubes disclosed in U.S. Pat. Nos. 4,349,598 and 4,895,428 (wherein one of the active cube faces is perpendicular to the reflector front surface) it is not possible to cut or rule a master containing all hexagonal or all rectangular microcubes by ruling straight lines in a single flat surface. Moreover, because of the geometric limitations inherent in ruling the cubes for the U.S. Pat. No. 4,349,598 and U.S. Pat. No. 4,895,428 patents, the cube structures disclosed therein are not useful where the primary light source will generally be at a near-zero incidence angle, such as in highway sign sheeting.
Processes for making tools having macrocubes are known in the prior art. Such tools are typically made by assembling a cluster of metal pins, each pin having a single cube corner machined and polished on one end. Hexagonal pins typically may have a dimension across parallel flats on the order of about 0.10 inch (2.5 mm). Rectangular pins have a short dimension of about 0.070 inch (1.8 mm) and a long dimension of about 0.120 inch (3.0 mm). A cluster of such pins is then used as a master to electroform a mold. These larger cubes, because of their height, are too large for use in the manufacture of thin flexible retroreflective sheeting requiring microcubes, but do find utility where larger (and thus taller) retroreflective elements are acceptable, such as in molded plastic reflectors for roadway markers, automobile taillights, and the like.
Because of manufacturing limitations, the smallest pin known to applicants has a cube shape about 0.040" square. Microcubes as used in flexible retroreflective sheeting generally are no greater than about 0.016 inch (0.4 mm) on a side, and in applicants' assignee's commercial sheeting products, the longest edge of the cube shape is about 0.010 inches (0.25 mm).
The term microcube (or a cube of small dimensions), has been used in patents of others to describe or claim sheeting products produced from tools made directly or indirectly from ruled masters, as opposed to retroreflector articles comprising macrocubes typically formed by grouping pins (or by other techniques used to form the larger cubes).
For tooling hexagonal cubes, an alternative to the "pin cluster" manufacturing technique is shown in Applied Optics, Vol. 20, No. 6, p. 1268, 15 Apr., 1981. It is there stated that one way to achieve hexagonal cube corners is to accurately machine and polish grooves in the edge surfaces of a stack of flat plates and to assemble the plates at a desired angle. The reference shows a photograph of several flat plates with grooves cut in one edge, stacked one atop the other and with adjacent plates shifted with respect to one another so that the grooves are offset. The tilted stack of plates so assembled results in a set of hexagonal cubes which may be used as a master for electroforming molds. However, this technique was disclosed decades earlier by applicants' assignee's founder and was stated to be an unsatisfactory technique for tooling retroreflectors, see U.S. Pat. No. 1,591,572 (FIG. 16, p. 5, 11. 85-99).
Heretofore, the above-described "stacked plates" method of forming macrocubes was not of practical interest for producing molds for retroreflective products on a commercial scale. First, the molds for macrocubes could be made satisfactorily by the aforementioned clustering of hexagonal pins. Secondly, as observed in U.S. Pat. No. 1,591,572, by using conventional machining and polishing techniques, it was not possible to cut and polish inside-intersecting faces with the precise angular tolerances and sharp edges achievable with the pin technique. In particular, any irregularities in the cube surfaces as might be caused by either the cutting operation or the polishing operation could disadvantageously increase the divergence of the retroreflected light and thus diminish the effective retroreflectivity of the cubes so formed. This recognized difficulty in polishing grooved internal angles is highly exacerbated with microcubes because the area that cannot be polished flat is a relatively greater percentage of the resulting cube face area.
As part of the present application, applicants disclose a technique for making and using thin plates that can be ruled without the need of polishing and that can be assembled in various ways to achieve microcube elements not previously available.
It is an object of the present invention to provide an array of microcubes which cannot be produced by ruling in one plane.
It is a further object of the invention to provide an array of microcubes in which the non-dihedral face-edges are not all parallel to a common plane.
It is still another object of the invention to provide means for interrelating three constructional parameters defining a hexagonal microcube (i.e., slippage, groove depth, and plate thickness, explained infra), by which the desired optical characteristics of the microcube can be optimized.
It is still another object of the invention to provide a retroreflective article and, in particular, retroreflective sheeting, having a pattern of hexagonal retroreflective microcubes having desired retroreflective characteristics.
It is another object of the instant invention to provide a method of making a tool including two or more contiguous hexagonal microcubes, which tool can be used for making a retroreflective article and, in particular, retroreflective sheeting.
It is still another object of the invention to provide a method of making a tool having a pattern of all hexagonal microcubes, which tool is made in part by ruling a set of grooves into the ends of a set of plates and then assembling the plates so as to define an array of hexagonal microcubes having desired retroreflective characteristics.
It is yet another object of the invention to provide an article having hexagonal microcubes wherein all of the cube faces are pentagonal; to provide a tool for making such an article; and to provide methods for making such an article and such a tool.
It is yet another object of the invention to provide a retroreflective article and, in particular, retroreflective sheeting, having rectangular retroreflective microcubes in which no dihedral face-edges of one cube are collinear with those of another cube, and in particular, such an article in which the microcubes provide desired retroreflective characteristics.
It is another object of the invention to provide a tool having a unique pattern of rectangular microcubes in which cube axis cant is not constrained by the need for collinearity of dihedral face-edges of adjacent cubes, which tool can be used for making a retroreflective article and, in particular, retroreflective sheeting.
It is another object of the instant invention to provide a method of making a tool having a pattern of rectangular microcubes in which dihedral face-edges are not collinear, which tool can be used for making a retroreflective article having rectangular microcubes, such as sheeting.
It is still another object of the invention to provide a method of making a tool having a pattern of rectangular microcubes, which tool is made in part by ruling grooves and bevels into plate ends to provide a desired rectangular cube shape and pattern.
It is also an object of the invention to provide a method of making rectangular microcube tools by means of assembling flat plates, on one end of which the rectangular microcubes have been formed.
It is still another object of the invention to provide an article having a pattern of retroreflective square microcubes, wherein the microcubes in a square set of four cubes have cube axes canted in four different directions.
It is yet another object of the invention to provide an article having a pattern of retroreflective pentagonal microcubes; to provide a tool for making such an article; and to provide methods for making such an article and such a tool.
It is still another object of the invention to provide an article having a pattern of pentagonal microcubes with canted cube axes, and such an article having pentagonal microcubes with differently canted cube axes, and tools for making such articles and methods for making such tools and articles.
Still a further object of the invention is to provide a retroreflective article having one or more triangular microcubes in which the cube shape and the position of the projection of the cube apex within the cube shape are independent of the cube axis cant.
Yet a further object is to provide such a retroreflective article in which adjacent triangular microcubes may have different degrees of inclination of the cube axes and are not necessarily matched pairs.
Other objects, advantages, and novel features of the instant invention will be understood by those skilled in the art from the following specification and the drawings appended hereto.